The present invention relates to an improved electrostatographic apparatus such as an electrostatic copying machine comprising improved means for generating and applying a bias voltage to a developing unit.
An electrostatographic apparatus of the present type is disclosed in U.S. Pat. No. 4,050,806 which is assigned to the same assignee as this application. An electrode is disposed closely adjacent to the surface of a photoconductive drum to sense the potential of an electrostatic image thereon. A voltage is induced in the electrode which is proportional to the electrostatic image potential and is fed to a computing circuit which generates a developing bias voltage in accordance therewith. The bias voltage which is applied to a developing unit is slightly greater than the potential of the background areas of the electrostatic image and prevents the formation of a gray background in the finished copy. Preferably, more than one electrode is provided and the minimum output of the electrodes is selected since it corresponds to the background area potential. A similar arrangement is disclosed in U.S. Pat. No. 3,892,481.
A problem which has existed heretofore regards the capacitances between the electrode, the drum and ground. Since the capacitance between the electrode and ground is comparable in magnitude to that between the electrode and the drum, only a fraction of the potential induced on the electrode appears at the input of an amplifier connected to the electrode. This is because the capacitances between the electrode, the drum and ground constitute a voltage divider.
Since the voltage at the input of the amplifier is only on the order of 1/100 or 1/1000 the magnitude of the electrostatic potential on the drum, a small error in the sensed voltage results in a large error in the applied bias voltage. This problem is compounded by temperature drifts and zero points shifts in the amplifier. A prior art expedient for compensating zero point shift is disclosed in U.S. Pat. No. 3,852,668 and comprises a shutter means provided to the electrode. Temperature drift has been reduced by providing the amplifier with stabilizing circuitry or mounting the amplifier in a constant temperature enclosure.
However, these expedients are only partially acceptable solutions to the problems.
Yet another problem is that the gap between the electrode and the drum is critical since it determines the capacitance between the electrode and drum and thereby the magnitude of the signal output. A large variation in signal output is produced by a small change in the gap.